1. Field of the Invention
The present invention relates to magnetic sensing elements that sense external magnetic fields as changes in resistance and methods for producing the magnetic sensing elements, and particularly relates to a magnetic sensing element with improved magnetic sensitivity stability and a method for producing the magnetic sensing element.
2. Description of the Related Art
FIG. 13 is a partial sectional view of a known magnetic sensing element (spin-valve thin-film element) which is taken in a direction parallel to the surface of the magnetic sensing element facing a recording medium.
In FIG. 13, a seed layer 2 made of a face-centered cubic (fcc) metal, such as NiFeCr, is formed on a base layer 1 made of Ta.
An antiferromagnetic layer 3, a pinned magnetic layer 4, a nonmagnetic material layer 5, a free magnetic layer 6, and a protective layer 7 are sequentially formed on the seed layer 2 to constitute a multilayer film T.
The protective layer 7 is made of Ta. The free magnetic layer 6 has a bilayer structure including a Co90Fe10 layer 6a and a NiFe layer 6b. The nonmagnetic material layer 5 is made of Cu. The pinned magnetic layer 4 is made of CoFe. The antiferromagnetic layer 3 is made of PtMn.
Electrode layers 10 are provided on both sides of the multilayer film T to allow a direct sensing current to flow in a direction parallel to the surfaces of the multilayer film T.
An exchange coupling magnetic field occurs at the interface between the antiferromagnetic layer 3 and the pinned magnetic layer 4 to pin the magnetization of the pinned magnetic layer 4 in a height direction (the Y1 direction in the drawing).
Hard bias layers 8 made of a hard magnetic material, such as CoPt, are formed on both sides of the free magnetic layer 6. The hard bias layers 8 generate a longitudinal bias field to align the magnetization of the free magnetic layer 6 in a track-width direction (the X1-X2 direction in the drawing).
An external magnetic field applied to the magnetic sensing element shown in FIG. 13 changes the magnetization direction of the free magnetic layer 6 relative to that of the pinned magnetic layer 4 to change the resistance of the multilayer film T. While a predetermined sensing current is allowed to flow through the multilayer film T, the magnetic sensing element senses the change in resistance as a change in voltage to detect the external magnetic field.
The free magnetic layer 6 of the magnetic sensing element shown in FIG. 13 has a bilayer structure in which the NiFe layer 6b is formed on the Co90Fe10 layer 6a. The NiFe layer 6b has lower coercivity than the Co90Fe10 layer 6a. 
Co90Fe10 has a high spin-dependent scattering coefficient, which increases with increasing amount of change in the mean free path of conduction electrons which occurs when the magnetization direction of the free magnetic layer 6 shifts from parallel to antiparallel to that of the pinned magnetic layer 4. That is, as the spin-dependent scattering coefficient of the material for the free magnetic layer 6 increases, the rate of change in magnetoresistance increases.
As shown in FIG. 13, the Co90Fe10 layer 6a of the free magnetic layer 6 may be formed on the nonmagnetic material layer 5 to increase the spin-dependent scattering coefficient of the free magnetic layer 6 and thus increase the rate of change in magnetoresistance. Such a magnetic sensing element is disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 2001-167410 (Patent Document 1, see Page 2 of this document) and 2003-174217 (Patent Document 2, see Page 3 of this document).
The rate of change in magnetoresistance of the magnetic sensing element may be increased by increasing the thickness of the Co90Fe10 layer 6a in contact with the nonmagnetic material layer 5. Simply increasing the thickness of the Co90Fe10 layer 6a, however, magnifies the magnetostriction of the Co90Fe10 layer 6a because Co90Fe10 also has a high magnetostriction coefficient λ, thus causing phenomena such as decreased output symmetry and increased Barkhausen noise. Such phenomena undesirably degrade the output stability of the magnetic sensing element.